Part II: 2D Density Profiles

Introduction

To demonstrate that BHNS mergers may form an SGRB engine under appropriate conditions, the final disk from the unmagnetized case was seeded with a purely poloidol magnetic field with an average magnetic-to-gas pressure ratio of ~0.1%. Though dynamically insignifigant initially, the strength of the fields is sufficient to resolve MRI throughout most of the disk due to the purely poloidal nature of the fields. Within ~.5 orbital periods, magnetic turbulence begins, accretion of the dipole poloidal fields occurs, and inflows diminish. The magnetic-to-rest mass density ratio,B2/2ρ0, amplifies to a maximum value greater than 100, and mildly relativistic outflows appear in funnels above and below the black hole.

B-Field Through Fixed Grid Points

In the clip, the rest-mass density of the star is plotted on a logarithmic scale normalized to the initial central density. the final disk from the unmagnetized case is seeded with a purely poloidal field with an average magnetic-to-gas pressure ratio of ~.1%. The following images display the density profile in a meridional plane intersecting the black hole center. Shown in white are streamlines of the projection of the magnetic field onto the meridional plane. Note that the streamlines of the full magnetic field do not lie in a single plane, but instead display a barber-pole like structure in the polar regions above the black hole. However, the streamlines of the projected field clearly display the collimated nature of the fields in the polar regions and the turbulent nature of the disk fields.

Velocity Vectors

In the clip, the rest-mass density of the star is plotted on a logarithmic scale normalized to the initial central density. the final disk from the unmagnetized case is seeded with a purely poloidal field with an average magnetic-to-gas pressure ratio of ~.1%. The following images display the density profile in a meridional plane intersecting the black hole center. Shown in yellow is the projection of the velocity field into the meridional plane, and as such, particles may be moving into or out of the plane at great speed, as is the case in the disk. The initial inflow in the polar regions soon becomes turbulent, and the velocities in these regions undergo a rapid 'turnabout', wherein a mildly relativistic outflow occurs in the funnels above and below the black hole.

Summary

Poloidal fields in the remnant disk can be amplified exponentially by MRI, driving the initial influx of matter from the disrupted neutron star to an outflow of matter in the regions above and below the black hole poles. The magnetic to rest-mass density ratio, B2/2ρ0, with an intial value of at most ~10-4, amplifies to a maximum value of over 100 in the funnels above and below the black hole. For steady-state Poynting-dominated jets, the energy-to-mass flux ratio, in this case approximately equal to B2/2ρ0, is equal to the maximum possible Lorentz factor in the asymptotic jet. At this time, the Poynting luminosity is 3.5×10-3 d/dt(M0c2) = 1.28×1052 ergs/sec, which may be sufficient to power an SGRB.